In some subsea wells, when the formation pressure is no longer sufficient to produce hydrocarbons, a technique called "gas lift" is employed to stimulate further production from the low-pressure formation. The gas-lift technique involves pumping, under pressure, gas into the annulus which enters the production string through gas-lift valves. The presence of gas in the tubing string reduces the weight of the column of fluid in the production string and allows the remaining formation pressure to move the hydrocarbons to the surface. Subsea wells that have their manifolds with access to the annulus installed below the waterline require connections, generally with divers or remotely operated vehicles (ROVs) in order to place the well on gas-lift service. For wellheads at substantial depths, the use of divers becomes impractical and the currently practical solution is to use ROVs.
Frequently, the access platform in an offshore location is a considerable distance from the actual subsea wellhead. The technique which is used to put the well on gas-lift service requires a connection of the gas source from the service platform to the wellhead. It is undesirable to allow liquids to get into this line since, when the well is put in gas-lift operation, the liquids will be displaced into the annulus and have a detrimental effect on downhole gas-lift equipment. Accordingly, one prior way to deal with this problem of liquid accumulating in the gas delivery line prior to connection to the subsea manifold was to put a valve at the manifold end of the gas delivery line and connect the gas delivery line using a diver who would then open the valve manually after connecting the line by inserting the stab. For locations where the manifold is at considerable depths, the use of a diver is impractical.
Another possibility would be to put the valve in the gas delivery line adjacent the stab and try to use the ROV to not only insert the stab but also to operate a valve on the fluid delivery line which comes out transversely from the stab. Because of the necessary configurations, it has not been practical to construct an ROV which has the capabilities of not only inserting the stab, but also operating a valve on an adjacent line.
To address the need for installation of a subsea gas-lift line without the risk of contamination of such line with seawater prior to its connection to the subsea manifold, the apparatus of the present invention has been developed so that the gas-lift line can be securely connected to a subsea manifold, as well as pressure-tested to a certain degree, while at the same time keeping the line free of seawater. This technique is possible without having to needlessly blow fluid through the line to try to keep seawater out of it. Such techniques become unworkable since fluid flow needs to be curtailed as the ROV inserts the stab into the manifold. At that point in time, seawater can back up into stab designs of the prior art. However, with the present invention, the stab and associated gas lines stay clear of liquids until the ROV secures the stab in the subsea manifold.